This invention relates to a method for producing low cost .alpha.-silicon nitride pre-shaped whisker compacts in which the whiskers are omni-directional and can be used as the high-modulus, high-strength reinforcing phase for metal, organic and ceramic matrix phase composites. The invention also relates to a method for producing .alpha.-silicon nitride whiskers for blending with metal, organic and ceramic molding powders to provide a high-strength, high-modulus reinforcing phase for composite parts produced from the blended molding powders.
Workers in the art have sought to produce reinforcement materials in the form of elongated, single-crystal fibres known as whiskers. The term whiskers generally applies to any single-crystal fibre having a thickness less than about 0.1 mm. and a length to thickness ratio of at least 100.
Whiskers are potentially important in the reinforcement of metals and other materials because of their shape, high-modulus of elasticity and high tensile strength. When dispersed in metals, the elongated shape of whiskers renders them difficult particles to displace to the grain boundaries, thus significantly increasing the creep resistance of the metal. Further, the high elastic modulus and tensile strength of many whiskers makes it possible for them to act as reinforcement for producing materials having superior strength to weight and stiffness to weight properties.
Workers are particularly interested in whiskers made from very stiff and light covalent compounds such as carbides, nitrides and oxides, which have elastic moduli higher than most metals and are also often many times stronger than steel, especially in proportion to their weight.
In general, workers have experienced extreme difficulties in successfully scaling-up laboratory methods for producing such whiskers. The yields and efficiencies of previous processes, moreover, have been found to be low. Further, the previous processes often produce whiskers in a variety of polymorphic forms, making it difficult to isolate a particular polymorph having a desired property such as thermal stability.
A specific compound that has been produced in whisker form is aluminum oxide (sapphire). The preparation of sapphire is difficult, however, because slight variations in process conditions greatly affect the quality of sapphire produced. The difficulties related to the preparation of sapphire whiskers have also been accompanied by prohibitively high costs.
Silicon carbide is another compound that has been produced in whisker form. Although produced at low cost, silicon carbide whiskers presently available are unfortunately contaminated by large quantities of non-fibrous particulate material. Silicon carbide, moreover, disadvantageously reacts chemically with some matrix materials and thus may fail to act as an independent reinforcing phase in such applications.
.alpha.-Silicon nitride is known to be a highly desirable reinforcement material which does not react chemically with matrix materials. Prior art attempts, however, to produce uncontaminated whiskers of .alpha.-silicon nitride have been extremely costly. Accordingly, the art has sought an improved method for producing .alpha.-silicon nitride whiskers and an improved method for dispersing the whiskers uniformly in a matrix phase.